In the digital transmission of binary information, the signals are generally converted to multilevel signals prior to transmission. The particular coding of the multilevel signal has a direct bearing on the bandwidth compression, the transmission efficiency, the cost and complexity of the equipment, the error performance and the difficulty of extracting clock or timing information.
For maximum efficiency, the multilevel symbol rate should be inversely proportional to the number of levels of the two signals. Thus, a 100% efficient quaternary code has a symbol rate equal to one-half the binary bit rate. If block mapping codes are used small coding blocks must be used during code conversion to reduce the complexity. In addition, the running digital sum of the transmitted signal should be constrained so that there is no d-c component, otherwise d-c restoration techniques will be required. The low-frequency power of the transmitted signal should also be small in order that small components (particularly small coupling transformers) may be used throughout the system and to minimize the effects of impulse noise. To permit simple clock extraction from the received signal, it is desirable that the spectral energy of the transmitted code be non-zero at the Nyquist rate and zero at twice the Nyquist rate. In addition, the transmitted code must contain sufficient framing and error checking information to function correctly in the transmission system irrespective of the input bit sequence.
In the past various forms of block coding have been used of which the 4B-3T, MS43 and FOMOT Block Ternary Codes are well-known examples. Comparison between these three codes is discussed in a paper entitled "Ternary Line Codes" by J. D. Buchner, Philips Telecommunication Review, Volume 34, No. 2, June 1976, pages 72-86. The MS43 Code is also described in U.S. Pat. No. 3,587,088 entitled "Multi-level Pulse Transmission Systems Employing Codes Having Three or More Alphabets" issued June 22, 1971 to Peter A. Franaszek. In these forms of coding, there is invariably a considerable loss in efficiency due to the use of multiple alphabets with resultant redundancy in order to constrain the running digital sum and hence eliminate the d-c component from the transmitted signal.
A highly efficient scheme is the duobinary system disclosed in U.S. Pat. No. 3,238,299 entitled "High-Speed Data Transmission System" issued Mar. 1, 1966 to Adam Lender. However, one major drawback with this system is that the spectral energy is maximum at d-c, thereby complicating the design of the repeater amplifiers used throughout the system. An alternate approach is described in U.S. Pat. No. 3,457,510 entitled "Modified Duobinary Data Transmission" issued July 22, 1969 to Adam Lender. While this coding scheme eliminates the d-c component, a null in the spectral energy occurs at both the Nyquist rate and twice the Nyquist rate so that complex clock recovery techniques are required.
An entirely different approach is described in U.S. Pat. No. 3,754,237 entitled "Communication System Using Binary to Multi-Level and Multi-Level to Binary Coded Pulse Conversion" issued Aug. 21, 1973 to Patrick de Laage de Meux. In this system, the binary signal is divided into words of n bits to which an (n+1)th bit of constant value is added before coding to a multi-level signal. The (n+1) bit words are then subdivided into partial words, each of which is translated into a multilevel pulse of one or the other polarity in order to constrain the running digital sum of the multilevel signal and hence eliminate the d-c component. Since the (n+1)th bit of each partial word is also inverted, this information can be utilized to correctly reconstruct the original word in the receiver. Also with this scheme, there is spectral energy at the Nyquist rate and none at twice the Nyquist rate thereby facilitating clock recovery. However, to obtain synchronization, an additional synchronization word is transmitted at periodic intervals. This synchronization word reduces the coding efficiency of this coding scheme over that which is obtained by adding only the (n+1)th bit of constant value to each word.